Electrochemical gas sensors are generally known. They usually have a plurality of electrodes, which are in conductive contact with an electrolyte liquid and form a galvanic element, hereinafter also called electrochemical measuring cell, in this manner. There are both sensors that are used stationarily and sensors that are used in portable devices. It is desirable, especially in case of stationary sensors, that they be constantly able to function over the longest possible period of time and be tolerant to fluctuations in the measuring range.
The technical field of use, especially for sensors with which nitrogen-hydrogen compounds can be detected, ranges, for example, from the chemical industry to agricultural plants over the monitoring of refrigerating plants. The sensors are used especially to recognize critical concentrations of flammable and/or toxic gases and to warn against a corresponding risk. The monitoring of the concentrations of ammonia (NH3), hydrazine and amines is of interest, in particular.
For example, EP 0 395 927 B1 discloses in this connection an electrochemical measuring cell for determining ammonia or hydrazine in a gaseous and liquid test sample, with at least one measuring electrode and a counterelectrode. To generate a reference potential for the determination of ammonia or hydrazine, a reference electrode, whose potential is used as a reference point for the measurement, is introduced into this measuring cell. EP 0 556 558 B1 also discloses such an electrochemical measuring cell for determining ammonia, amines, hydrazine and hydrazine derivatives.
The detection of the nitrogen-hydrogen-containing compounds, e.g., ammonia, various amines or hydrazine, typically takes place in such measuring cells by means of a corresponding reaction between the gas flowing into the sensor, the electrodes and the electrolyte of the sensor.
However, various problems may arise in such prior-art electrochemical measuring cells. For example, protons released at the working electrode may lead to a lowering of the pH value at the electrode. It may happen in such a case that more ammonia is oxidized only when the protons have been successfully neutralized. It may thus happen after a certain time of continuous gas admission that the measured signal drops (collapses) despite constant gas concentration.